Present hard disc drives (abbreviated as HDDs) include one or more magnetic discs that use a magnetic film as a recording medium. The HDDs are typically used as a storage device for a computer.
In HDDs, the recording/reproducing data is performed by a magnetic head. The magnetic head is floated from the surface of a rotating magnetic disc away from the surface of the magnetic disc with a gap of several tens nm using a floating mechanism (slider). Bit information on the magnetic disc is stored in the data tracks arranged concentrically on the medium, and the magnetic head is moved and positioned to a target data track on the surface of the recording medium at a high speed to record/reproduce data. A positioning signal (servo signal) is concentrically written to the magnetic disc to detect the relative position between the magnetic head and the data tracks. The magnetic head detects the position thereof at a fixed time interval. A servo signal is written to a magnetic disc using a dedicated device called a servo writer so that it does not deviate from the center of the magnetic disc (or the center of the locus of the magnetic head) after the magnetic disc is installed in HDD.
The recording density of the magnetic recording medium, such as a magnetic disc or the like, at the present developing stage reaches 100 Gbis/in2, and the storage capacity thereof is increased about 60% per year. In connection with this, there is a tendency for the density of the servo signal for positioning the magnetic head to be increased, while the writing time of the servo signal is increased year by year. The increase of the writing time of the servo signal becomes one critical factor that reduces productivity of the HDD and increases the cost thereof.
In comparison to the servo signal writing system using the magnetic head of the servo writer described above, the writing time of the servo signal can be dramatically shortened by collectively writing the servo signal through a magnetic transfer technique, as disclosed for instance in JP-A-2001-283433. FIGS. 1A–B and 2A–2C illustrate the principle of this magnetic transfer technique. The magnetic transfer is divided into an initial demagnetization step and a transfer pattern writing step.
FIG. 1A shows the initial demagnetization step. FIG. 2A shows a movement path of the magnetic core 14. Here, the magnetic layer 13 is uniformly magnetized in the circumferential direction. A magnetic core 14 having a permanent magnet 15 is moved in the direction of the arrow (pointing right) while it is floated above the surface of a magnetic disc 11 having a magnetic layer 13 on a substrate 12 at a fixed gap or interval (1 mm or less). Initially, the magnetic layer 13 formed on the substrate 12 is not uniformly magnetized in a single direction, but is magnetized in the uniform direction by the magnetic field leaking from the gap 16 of the magnetic core 14. The arrows in the magnetic layer 13 represent the magnetization direction, which is the opposite of the movement direction of the magnetic core 14.
FIG. 1B shows the transfer pattern writing step. FIG. 2B shows a magnetic transfer master disc (master disc) 17 disposed and positioned on the magnetic disc 11. FIG. 2C shows a movement path of the magnetic core 14. The master disc 17 is brought into close contact with the magnetic disc 11, and the magnetic core 14 is moved along the circumferential direction. The servo signal embedded as a magnetization pattern in the master disc 17 is transferred onto the magnetic disc 11 using the magnetic transfer technique described above. The master disc 17 has a soft magnetic layer 20 embedded in a magnetic layer 19 on a silicon substrate 18. In the soft magnetic layer 20, a servo signal to be recorded in a magnetic recording medium is embedded as a magnetization pattern. The magnetic core 14 magnetizes the magnetic disc 11 in the opposite direction to the direction of the magnetic field in the initial demagnetization step through the master disc 17. At this time, magnetic field leaking from the gap 16 infiltrates into the master disc 17 and reaches the magnetic disc 11 in the areas having no soft magnetic layer 20 to magnetize the magnetic layer 13. On the other hand, in the areas having the soft magnetic layer 20, the magnetic field passes through the soft magnetic layer 20 serving as a magnetic path having small magnetic resistance. Thus, the areas below the soft magnetic layer 20 is shielded and thus not written.
FIG. 3 shows the format of the conventional servo signal. The servo signal comprises a servo AGC 31, a servo detection pattern 32, which is a specific pattern for identifying the servo signal, cylinder information 33, sector information 34, and servo burst information 35. The format of the servo signal will be described in detail.
The servo AGC (Automatic Gain Control) 31 is associated with an AGC circuit of an amplifier for amplifying a signal read from a magnetic head. Normally, the AGC circuit of the amplifier operates ordinarily under the assumption that data are written in portions other than the servo signal. However, under the state where only the servo signal is written and no data are written, or under the state where the servo signal is read immediately after the data are written, the gain of the amplifier is approximately kept to the maximum, and thus it is impossible to read the servo signal normally. Therefore, a dibit pattern of all “1” bits, that is, a signal having a fixed frequency (hereinafter referred to as servo frequency) is first written in the servo signal. The gain of the amplifier is returned to the normal level by reading the servo AGC 31. A PLL (Phase Locked Loop) circuit for generating clocks to read the servo signal is synchronized with the servo frequency. About 100 bits are formed as the servo AGC 31.
The servo address information comprises the cylinder information 33 and the sector information 34. The cylinder information of servo tracks is written in the cylinder information 33 while subjected to gray coding. The HDD positions the head to a target track based on the cylinder information 33 and read or writes data there. The gray coding will be described later with reference to FIGS. 4A and 4B. The sector information 34 is information relating to the sectors in the tracks. The sector is an area formed by dividing a track into several hundreds parts, and it serves as a data recording/reproducing area in the HDD. A sector address is normally represented by binary numbers. For example, when 90 sectors are formed on the whole periphery, the sector address has a 7-bit length.
The servo burst information 35 is information to position the magnetic head on a target track after the magnetic head is moved to the target track. In general, the magnetic head is positioned to the center of the target track by comparing the signal amplitude of signals deviated in phase by 180°. The pattern shown in FIG. 3 is a burst pattern in which A, B, C and D are deviated in phase by one track width.
FIGS. 4A and 4B show the gray coding of the conventional cylinder information. FIG. 4A shows a binary gray code conversion rule of 4 bits. FIG. 4B shows a gray code conversion rule in the cylinder information of 19 bits. Only variation of one bit is made between neighboring cylinders by the gray coding. Accordingly, even if any one bit is erroneously read when the cylinder information is read by the magnetic head, the magnetic head is prevented from accessing the correct position. For example, the recording range of a 3.5-inch type magnetic recording medium is equal to about 29,000 tracks per surface when the radius is set to 17.85 to 47.00 mm and the track width is set to 0.1 μm. When three double-sided recordable magnetic recording media are used in the HDD, the total number of cylinders is equal to 2×3×29,000=174,000, which corresponds to 18 bits. When the servo bit length at the radius of 17.85 mm is set to 0.1 μm, the total servo bit length is equal to 1.8 μm and the servo bit length at the radius of 23.5 mm is set to 0.13 μm, the total servo bit length is equal to 2.4 μm. This means that a section in which “0” or “1” continues exists over the length corresponding to the total servo bit length in a specific servo pattern.
A data pattern exists in the magnetic recording medium in addition to the pattern of the servo signal described above. The data pattern is recorded by using an RLL (Run Length Limited) code. The RLL code is represented by using as parameters the minimum magnetization inverted interval: d, the maximum magnetization inverted interval: k, the bit length of original data: m, and the bit length after modulation: n. FIG. 5 shows the conversion rule of an RLL 1-7 code of d=1, k=7. The RLL 1-7 code means that the minimum and maximum values of the magnetization inverted interval are equal to 1 and 7 respectively, the data sequence of 2 bits is converted to 3 bits in principle, and the data sequence of specific 4 bits is converted to 6 bits. FIG. 6 shows a conversion example of the data sequence based on the conventional RLL 1-7 code. The RLL 1-7 code is designed so that one “0” at minimum and seven “0” at maximum exists between “1” and “1,” and “0” or “1” does not continue over a long section. Since bit length is set to 0.1 μm, a section in which “0” or “1” continues is equal to 0.7 μm.
When the servo signal is written using the magnetic transfer technique described above in the 3.5-inch type magnetic recording medium, the magnetic transit interval of the servo address information, that is, the section where “0” or “1” continues covers a broad range from the data bit length of 0.1 μm to the total servo bit length of 2.4 μm, which corresponds to about 24 times. FIGS. 7A–7C show the version in magnetic flux density of the magnetic layer when the length and period of the soft magnetic layer are varied. FIG. 7B, illustrates a case where the length W of the soft magnetic layer 20 is equal to 0.7 μm and the interval P is equal to 1.4 μm and FIG. 7C illustrates a case where the length W is equal to 2.0 μm and the interval P is equal to 4.0 μm. The magnetic flux density at the lower portion of the soft magnetic layer 20 is larger when W=2.0 μm and P=4.0 μm. Ideally, the magnetic flux density of the magnetic layer 13 of the magnetic disc 11 at the lower portion of the soft magnetic layer 20 is preferably equal to zero. If the length W of the soft magnetic layer 20 is increased and the interval P of the soft magnetic layer 20 is increased, the magnetic flux of the recording magnetic field flowing into the soft magnetic layer 20 is increased, and the magnetic flux density of the soft magnetic layer 20 is increased, so that the magnetic flux density exceeds the saturated magnetic flux density of the soft magnetic layer 20, and the magnetic field leaks into the magnetic layer 13 of the magnetic disc 11.
FIGS. 8A–8C show the variation in magnetic saturation point at the lower portion of the soft magnetic layer when the length and period of the soft magnetic film are varied. It is apparent that magnetic field leaks due to magnetic saturation. On the other hand, the magnetic flux density in the gap between the two soft magnetic layers 20 is smaller when W=2.0 μm and P=4.0 μm. This is because when the interval between the two soft magnetic layers 20 is large, the magnetic fluxes passing through the soft magnetic layers 20 pass nearer to the magnet side, so that the magnetic flux density in the magnetic layer 13 of the magnetic disc 11 is reduced.
As shown in FIGS. 1A–1B, the magnetic transfer is carried out in both the initial demagnetization step and the transfer pattern writing step. To ensure a complete magnetic transfer on the whole surface of the magnetic disc, it is necessary to reduce the leakage of magnetic flux density at the lower portion of the soft magnetic layer and increase the magnetic flux density between the soft magnetic layer and the soft magnetic layer. As shown in FIGS. 7A–7C and 8A–8C, when the length of the soft magnetic layer is increased and the interval is also increased, the leakage of the magnetic flux density at the lower portion of the soft magnetic layer is increased, and the magnetic flux density is reduced between the soft magnetic layers. As a calculation example, the difference corresponds to about three times in the length W of the soft magnetic layer and twice in the interval. However, when the magnetic transit interval of the servo address information covers a broad range of about 24 times, this tendency is further remarkable. That is, some portion is not subjected to magnetization inversion, and thus it is difficult to fully carry out the magnetic transfer.
The present invention has been implemented in view of the foregoing problem. There remains a need for a master disc that can carry out magnetic transfer with high reliability. The present invention addresses this need.